You need to check the Apply Default Animation checkbox on your armature if you want to play animation on it, but non-uniform scale is not supported, so you'll not see the animation. You can't use scale only by one axis, you need to use all three axes.

Mikhail Luzyanin: Thanks, fixed it now.I had failed to spot the significance of the bit about non uniform scaling in the docs. For anyone who's interested I got around it by using separate bones which are just moved (location) instead of scaled to get the desired effect. Thanks again. P